Repair of Tank Bottoms in Atmospheric Storage Tanks | Carmagen Engineering
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Repair of Tank Bottoms Bottoms in Atmospheric Atmospheric Storage Tanks By Ray Chao
Tank bottoms in atmospheric atmospheric storage tanks tanks (ASTs) are susceptible susceptible to both external and internal corrosion, corrosion, and an earlier article discussed discusse d tank bottom inspection and evaluation. Repair of corroded tank bottoms in old ASTs is thus a common maintenance maintenance undertaking undertaki ng by tank owners. API Standard 653, Tank Inspection, Inspection, Repair, Alteration Alteration and Reconstruction, Reconstruction, provides guidance on the repair of tank bottoms in ASTs. The rules in API 653, however, contain certain restrictions restrictions on weld repairs and, in some cases, require that the shell penetrations penetrations be raised. This article will examine these these requirements requirements and discuss safe repair of tank bottoms bottoms without undue costs costs or prolonged tank downtime downtime..
Repairing a Portion of the Bottom API 653 restricts welding in the “critica “criticall zone” when making repairs to a portion portion of the tank bottom. The “critical zone” is defined defined as: within the annular annular plate ring, within within 12 inches of the shell, shell, or within 12 inches inches of the inside edge of the annular annular ring. No welding, welding, welded-on welded-o n patch plates, or weld overlays overlays are permitted permitted within the “critical zone” zone” except for welding welding of widely scattered scattered pits, cracks in the bottom plates, the shell-to-bottom shell-to-bottom weld, or or where the bottom bottom or annular plate is being replaced. replaced. If more extensiv extensive e repairs are required,, the bottom plate under the tank shell would have to be cut out and a new plate installed. required The “critical zone” is supposed supposed to be based on stress consideration considerations. s. It is well known known that the maximum maximum stress stress in a tank bottom bottom exists at the toe of the inside shell-to-bottom fillet fillet weld at the annular plate or sketch plate. However, However, this is a very localized bending stress and it decays rapidly with distance distance from the shell-to-bottom shell-to-bottom junction. Indeed, the annular plate width requirement requirement of API 650 was established establish ed by determining the required distance for the edge moment to damp out complete completely ly using the beam on an elastic foundation foundatio n approach. Therefore, Therefore, the "critical zone" should only be the immediate area of the shell-to-bottom junction. junction. Yet, API 653 does not prohibit repair repair of the shell-to-bottom shell-to-bottom weld. Weld repairs in the the bottom “critica “criticall zone” should be treated treated no differently than than those permitted permitted during construction construction of new tanks. The weld restrictions restrictions in the “critical zone” zone” are unwarranted, unwarranted, except that if welded-on patch patch plates are used, they should should be located several several inches away from the shell-to-bottom shell-to-bottom fillet weld in order to minimize the compound compound effect of two stress risers located located near each other. All weld repairs within within 12 inches of the shell-to-bottom shell-to-bottom junction should should be examined by the magnetic particle particle method. The following following provides examples of what should be permissible. More than 25 years ago, the author led an effort to relocate two new 600,000 Bbl (260 ft. diameter by 64 ft. high) floating floating roof tanks tanks due to foundation foundation failures that occurred when when the tanks were undergoing undergoi ng hydrostatic tests. tests. The moves were successfu successfully lly accomplished accomplished by using the water flotation method which subjected subjected the tank bottoms to much much higher stresses than those those under normal normal operating conditions. conditions. In order to protect the lap weld between between the annular annular plate and the bottom plate from potential failures, failures, 6-inch wide by 1/4-inch thick patch plates covering covering the lap weld over its entire circumference circumfe rence were installed. installed. The tanks were subsequently subsequently placed into service without any ill effects. It may also be noted that weld overlay to restore base metal thickness thickness for strength considerations considerations is permitted by the ASME B&PV Code Section VIII for pressure vessels.
Complete Bottom Replace Replacement ment When a tank bottom is corroded beyond repair, a new bottom must often be installed over the existing corroded bottom. This is done by slotting the new bottom through the tank shell several inches above the existing bottom. The rules in API 653 require the spacing between the existing existing welds around around nozzles and the new bottom-to-shell bottom-to-shell weld to comply with the requireme requirements nts of API 650. Trimming of the existing nozzle nozzle reinforcing plate is permitted to satisfy the weld weld spacing requirements, requirements, provided that the modified detail complies complies with the requirements requirements of API 650. In many instances, these requireme requirements nts would necessitate raising raising the existing shell nozzles. With an API “Regula “Regularr Type” nozzle, if the weld spacing spacing requirements requirements cannot be met met by trimming the existing reinforcin reinforcing g plate, the lower half of of the existing reinforc reinforcing ing plate can be removed removed and replaced with a new plate plate to form a “tombsto “tombstone” ne” shaped reinforcing reinforcing plate. plat e. In essence, essence, this this converts converts the the “Re “Regula gularr Type Type”” nozz nozzle le to a “Lo “Low w Type” nozz nozzle. le. Howeve However, r, with an API API “Lo “Low w Type” Type” nozz nozzle, le, the distance from the tank bottom bottom to the center of the nozzle would be reduced to less than the minimum required required by API 650 and the nozzle would have have to be raised. Raising existing existing shell nozzles involves involves cutting and welding welding of shell plates which may be of unknown unknown toughness, toughnes s, thereby increasing increasing the risk of a brittle fracture. One must therefore therefore examine examine the technical justification justification for raising nozzles. nozzles. The reason for raising nozzles is to comply with the nozzle details given in API 650 which provide reinforcement reinforcement of shell openings generally generall y in accordance with the well-established area replacement replacement rules of the ASME Code. However, nozzles nozzles in an AST that are located near the tank bottom are subject to much lower stresses stresses under the hydrostatic hydrostatic head pressure than those at higher evaluations. evaluatio ns. In fact, because the bottom of the shell is restrain restrained ed from radial displacement by the bottom plates, the circumferential circumferential membrane stress stress in the tank shell at the bottom of the tank is essentially zero. zero. Therefore, the requireme requirements nts for nozzle reinforcement reinforcement should be based on stress analysis, analysis, consideri considering ng the cross-sectional cross-sectional area available in the shell plate for reinforce reinforcement, ment, as allowed by the ASME Code, as well as by API 650. In most cases, there should should be no need to raise the nozzles.
Summary In summary, the API 653 restrictions restrictions on weld repairs and requirements requirements for raising shell penetrations penetrations when making tank bottom bottom repairs should be reexamined. reexamined. Relaxatio Relaxation n of these rules should be possible to facilitate facilitate tank bottom repairs without affecting affecting tank safety.
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18-Nov-11
